QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett (“the '410 patent”) and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33 (“Bouwmeester”), which is incorporated by reference herein by way of background information.
The typical so-called “one way” QKD system such as disclosed in the '410 patent, use a “shared interferometer” that includes two interferometer halves, with one half at Alice and one half at Bob. Because the two interferometer halves are located remote from each other, differences in the optical path length of the interferometer halves can arise from local environmental effects. A difference in the optical path length, known as “phase error,” reduces the interference visibility (“system visibility”) of the single-photon-level optical pulses (“quantum pulses”), which is detrimental to the efficiency of the QKD process.
Accordingly, the typical one-way QKD systems need to be actively stabilized in order to maintain the optical path-length balance of Alice and Bob's shared interferometer to within a fraction of the wavelength (e.g., ˜30 nm for 1.5 μm light). This can be done, for example, by passing “classical” pulses (i.e., multi-photon optical pulses) through the shared interferometer at one QKD station (e.g., Alice) and detecting it at the output of the other QKD station (e.g., Bob). The QKD system is thus configured so that the classical optical pulses follow the same optical path traversed by the quantum pulses. Consequently, it is possible to monitor the phase error superimposed upon the qubits by observing the interference of the classical signals at the output of the interferometer. Using error signals generated by these interference patterns, it is possible to produce negative feedback for an actuator adapted to counteract this phase error. In response to the feedback signal, the actuator creates a compensating phase change at a single location (e.g., at Bob) to restore the optical path length balance. An example of an actively stabilized one-way QKD system is described in WIPO PCT Patent Application Publication No. WO2005067189 A1, entitled “Active stabilization of a one-way QKD system,” published on Jul. 21, 2005, which patent application is incorporated by reference herein.
When faced with high-frequency or high-amplitude disturbances, the feedback system's tracking range and bandwidth limitations may allow the shared interferometer to momentarily (e.g. over multiple qubit intervals) fall out of balance. In some cases these limitations are manifested by operating near the limits of the actuator range. Due to the periodic behavior of optical interference, the actively stabilized interferometer may have several stable modes of operation. Thus, in the case where the operating range of the actuator covers multiple modes, one solution to contending with limited actuator range is to “reset” the actuator to the center of its operating range when it comes too close to one of the range limits. However, depending on the implementation of this process and/or the speed of the actuator, resetting may cause a sudden spike (“glitch”) in phase error as the actuator hops across several modes and re-stabilizes itself at a mode near the midpoint of its operating range. These phase errors in turn are projected onto the quantum signal and thus leads to increased QBER.
Ideally, the control system used in one-way QKD systems will have sufficient bandwidth and range to eliminate any and all phase perturbations that arise. Realistically, this may not always be possible because there are likely other design tradeoffs that prevent such ideal performance. Among these are the insertion loss of these components, the permissible power level of the classical feedback signal, and cost restrictions. It is also conceivable that the QKD system may be employed in an environment where it is subjected to somewhat frequent high amplitude vibrations, such as a mobile military platform. In all of these cases the feedback system alone may not be capable of continuously eliminating the time-varying phase error. This is significant because each erroneous quantum signal bit leads to the loss of multiple error-free bits that are consumed in the error correction and privacy amplification processes.